1. Field of the Invention
The present invention relates in general to differential amplifiers and in particular to a differential amplifier having an adjustable common mode output voltage.
2. Description of Related Art
FIG. 1 illustrates in schematic diagram form a typical prior art differential amplifier 10 including a pair of transistors Q1 and Q2, a pair of matching load resistors R1 and R2 linking collectors of transistors Q1 and Q2 to a voltage source V.sub.D, a pair of matching current sources I1 and 12 supplying emitter current to transistors Q1 and Q2, and a resistor R3 connected between the emitters of transistors Q1 and Q2. Amplifier 10 amplifies an input signal V.sub.IN applied across the bases of transistors Q1 and Q2 to produce an output signal V.sub.OUT across the collectors of transistors Q1 and Q2.
The common mode output voltage of a differential amplifier is defined as the average voltage at its differential output terminals. For the amplifier 10 the common mode voltage V.sub.CM is a function of a load current and a resistance as follows: EQU V.sub.CM =V.sub.D -I.sub.L R.sub.L
where load current I.sub.L =I1=I2 and load resistance R.sub.L =R1=R2.
In many applications it is desirable to precisely set the common mode output voltage of a DC-coupled differential amplifier. From the above equation we can see that we can choose the common mode voltage by appropriately adjusting the values of load resistors R1 and R2, the magnitude of load currents I1 and I2 and the magnitude of supply voltage V.sub.D. Thus when we design a differential amplifier we choose the values of load resistors R1 and R2 and load currents I1 and I2 in part to set the desired common mode voltage. However since process variations cause the actual resistance of resistors or current output of current sources to vary somewhat from their nominal values, the actual common mode voltage will vary somewhat from the design value. Load resistors R1 and R2 have been implemented using adjustable resistors, such as for example potentiometers or laser-trimmable resistors, when a more precise control over common mode voltage has been needed. The resistance of the load resistors can then be iteratively adjusted while measuring the actual common mode output voltage so as to more precisely set the common mode voltage to a desired level. similarly when current sources I1 and I2 are adjustable or when voltage source V.sub.D is adjustable, we can iteratively adjust the current produced by current sources I1 and I2 or iteratively adjust the voltage source until we obtain the desired common mode voltage.
However it is inconvenient and time-consuming to measure the common mode output voltage of a differential amplifier while iteratively adjusting its load current, load resistance or supply voltage. And even though we can precisely adjust an amplifier's common mode output voltage by such methods, we cannot be certain that the amplifier's common mode output voltage will stay where we set it. The resistance of the load resistors R1 and R2, and the current produced by current sources I1 and I2, and the source voltage V.sub.D produced by many power supplies are temperature-dependent. Thus when the temperature of amplifier 10 changes, so too does its common mode output voltage.
What is needed is a differential amplifier having a common mode output voltage that can be conveniently and accurately adjusted to a desired level that is stable irrespective of process and temperature variations.